For those who manually compiled MySQL, the default
mysqlnd settings may results in error.
Your mysql_error() function may give you this message:

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No such file or directory (trying to connect via unix:///tmp/mysql.sock)
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This is because your manually compiled MySQL has its own place for socket.
You need to provide PHP a reference for an appropriate mysql socket.
That means you need to edit php.ini and add a section like this:

One important part of the MySQL Native Driver, which is overlooked, is that when connecting to MySQL via a socket, the connection is subject to the "default_socket_timeout" value.

This means that if you have the default_socket_timeout set to 60 (default value), and you have a query that takes longer than 60 seconds to respond, the connection will be severed and getting the query results will fail.

The solution (as of PHP 5.3.0) is to change default_socket_timeout to -1 in php.ini, or with ini_set.